Heating Basics — Limit Switches

Limit switches are one of the primary safeties in any piece of heating equipment, regardless of whether it is a furnace, air handler with electric heat, or packaged unit.  Anytime there is heat as part of a system, there is going to be a limit switch to protect the unit from over heating conditions.

Limit switches can come in a variety of shapes :

To often, these switches get replaced without, first, finding out why the limit switch was tripping. The tech see a “flash code” on a control board saying the limit switch tripped and then goes and changes it.  Keep in mind, that “flash codes” are a diagnostic only and are there to let you know that something has happened.  Today’s tech still needs to perform the complete diagnosis to determine why the “flash code” appeared on the control in the first place.

Nuisances limit tripping can be caused by any number of reasons and the cause needs to be determined to assure that the unit is operating safely and will not continue to have the unit cycle on limit.  Here is a list of item to be checked to prevent future problems with this tripping:

  1. Determine what Gas is being used and is the furnace properly setup for that fuel (propane or natural):
    1. Correct orifices installed per unit ‘Rating Plate’
    2. Gas valve converted properly: Refer to valve manufacturer for details.
  2. Check Manifold Pressure to insure it is within range of furnace ‘Rating Plate’:
    1. Usually 3.5 IWC for Natural Gas and 10.0 IWC for LP.
    2. If the unit is multi-stage make sure you do this on HIGH FIRE or full input.
  3. Check furnace temperature output with ‘Rating Plate’ maximum temperature limits:
    1. Temperature Rise across the heat exchanger.
      1. measure the return air temperature
      2. measure the supply air temperature at least 6 feet down from the furnace to avoid picking up radiant heat off the heat exchanger or evaporator coil.
      3. If the unit is multi-stage make sure you do this on HIGH FIRE or full input
  4. Check the Discharge Air Temperature and make sure it is within the name plate rating of the equipment.
    1. If the unit is multi-stage make sure you do this on HIGH FIRE or full input
  5. Check blower motor speed selection per installation instructions and wiring diagram for proper cfm.
    1. Confirm control board has energized correct blower output; i.e. ‘HEAT’.
    2. Check amperage reading of motor compared to ‘Rating Plate’. If the motor is exceeding it’s amperage rating, then:
      1. Check the capacitor to make sure it is within tolerances of its rating.
  6. Check blower wheel and housing:
    1. Dirt on blower wheel vanes reduces airflow
    2. Check that the blower wheel is centered in housing and set screw is tight.
  7. Check for dirt on secondary heat exchanger fins of 90%+ furnaces which can reduce heat transfer, clean as necessary.
  8. Check Total Static Pressure. Most furnace manufacturers rated cfm is based on being at or under 0.5 IW

One test that is often overlooked is clocking the gas meter. By knowing the BTU content of the gas being supplied and then seeing how much gas is being used by the furnace, you can verify that the unit is not over fired or under fired. Setting correct manifold pressure does not do this. If there is just 1 oversized orifice, the unit is going to be overfired even at 3.5 IWC manifold pressure.

Limits are safeties and are not  designed to be operating controls. They are present to keep the unit operating in a safe manner.  Under normal circumstances, the unit should not be operating or cycling on the limit control.

Hopefully this post will help in diagnosing units that are experiencing nuisance limit cycling or tripping.


About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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3 Responses to Heating Basics — Limit Switches

  1. tony says:

    Mike, you sure know what you’re talking about! It’s been very educational to read your posts. Thanks and keep up the good work!

  2. Tim G. says:

    Mike thank you for all info and hard work.

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